The invention relates to a method of s partial discharge pulses originating from various partial discharge sources in an electric system and preferably of determining the number of the partial discharge sources.
The voltage strength of an insulating structure refers to its ability to endure voltage stress without electric discharges that cause disturbances or damage. If the voltage stress in an the insulating structure is increased sufficiently, discharges occur which make the insulation completely or partially conductive. The latter are called partial discharges. A partial discharge does not unite the electrodes and, thus, the insulating properties of the insulating material do not completely disappear. Partial discharges do, however, wear the insulating material and thus further weaken its voltage strength and may finally lead to a complete electric discharge. Partial discharges can be divided into two main groups, internal and external discharges. Internal discharges comprise cavity discharges and external discharges comprise surface, corona and spark discharges. Each group can further be divided into several subgroups which are often difficult to clearly distinguish from each other. Partial discharge pulses are very fast pulses and usually occur as pulse groups. A partial discharge and the reversal of charge that occurs in connection with it show as a current pulse in the connectors of the insulating material. In practice, these current pulses also sum into the phase voltage of the system.
There are several known solutions for detecting partial discharges in an electric system. Partial discharges can, for instance, be detected by means of an electric measurement, acoustically by the sound they generate or on the basis of the electromagnetic radiation they produce. To detect the existence of partial discharges, it is also useful to distinguish from each other the partial discharge pulses originating from different partial discharge sources. If the partial discharge pulses originating from different sources can be classified into their own groups, identifying the cause of the partial discharge becomes easier, as the partial discharge sources can be analysed one at a time and it is also possible to determine the number of the partial discharge sources. Partial discharges can be generated simultaneously in several partial discharge sources, but known solutions for detecting partial discharges usually cannot distinguish from each other partial discharges coming from different sources.
It is thus an object of the invention to develop a method and a system implementing the method so as to solve the above-mentioned problems. The object of the invention is achieved by a method and system characterized in what is stated in the independent claims 1 and 9. Preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on classifying partial discharge pulses originating from different partial discharge sources in an electric system into pulse groups on the basis of one or more characteristic parameters defined from them. In addition, the number of the partial discharge sources can be defined by means of the pulse groups.
The method and system of the invention provide the advantage that they do not require any training phase performed in advance, but allow the distinguishing from each other of partial discharge pulses originating for instance from different partial discharge sources of different types or at various distances to the extent that they differ from each other within the scope of the used characteristic parameters. In addition, the method and system of the invention facilitate the identification of the cause of a partial discharge by enabling the analysis of partial discharge sources one at a time and also enable the automatic determination of the number of the partial discharge sources.